1. Field of the Invention
The present invention relates to a metal gasket in which a shim is laid on a bead plate.
2. Description of the Prior Art
Conventionally, there has been well known a metal gasket having a shim or spacer to increase a thickness of an area around a bore aperture. Such metal gaskets are disclosed in, for example, Japanese Patent Laid-Open Nos. 155375/1987 and 219572/1992.
Moreover, a metal gasket in which a thick member or shim is interposed between base plates is disclosed in a variety of prior documents, for example, Japanese Patent Laid-Open Nos. 175579/1992, 219572/1992, 207672/1994, 110827/1998 and 241769/1999.
As the metal gaskets disclosed in the prior documents enumerated above are of a type in which no fold is prepared around the bore aperture, a shim of uniform thickness is arranged on the base plate or interposed between the confronting base plates around the bore apertures to help ensure the compensation for difference in sealing stress, which might otherwise occur between the areas around the bore apertures and other major area including therein water holes, oil openings and the like. In addition, any metal gasket of the type disclosed in the documents recited above employs a thick spacer disposed between the base plates or a shim of uniform thickness secured on the base plate, thereby to compensate for irregularities in the mating surfaces and/or local reduction in sealing stress, which might be caused between the confronting surfaces of the cylinder head and cylinder block owing to the repeated expansion and contraction.
Disclosed in Japanese Patent Laid-Open No. 265022/1994 is a metal gasket in which an auxiliary plate or intermediate shim disposed between the confronting base plates extends around the bore apertures and has a peripheral dimension that is varied within a range of the width across the bead according to the gasket design requirements and further the bead is formed linearly at an area between the adjoining bore apertures. In the prior metal gasket constructed as described just above, the perimetric edge of the auxiliary plate always exists in the width across the bead on the base plate and also the auxiliary plate is constant in its peripheral dimension or diametric width around the bore aperture, which is defined depending on the gasket design requirements. No auxiliary plate extends over the entire width across the associated bead. Besides, when having squeezed strongly the prior metal gasket in which the auxiliary plate extends to cover the major of the width across the bead, the single bead is deformed compressively into two rows of bead, thus resulting in increasing the contacting area with the base plates to enhance the tight sealing around the bore apertures.
Either as described just above or as shown in FIG. 20, any adjoining shims 2P laid on a bead plate 1P around bore apertures 3P in most prior metal gasket come in joining with each other at their merging areas 4P, so that no shim varies in width continuously around the associated bore aperture. With the shim described above, it is very hard to make uniform the sealing stress throughout around the bore aperture, especially at areas neighboring bolt holes 5P, in some tightening conditions of the cylinder head to the cylinder block. Thus, such prior metal gasket has a tendency to less flexibility or adaptability on a practical application to a variety of engines, which are diverse in tightening condition. The closed-deck type engine, as mostly great in rigidity of the cylinder block, has the tendency to somewhat higher tightening or hold-down load, compared with that in the open-deck type engine. This causes the sealing stress around the bore apertures a local rise at the areas nearby the tightening holes, thus making it tough to ensure the sealing stress that is uniform throughout around the bore aperture. In order to cope with the drawback in the sealing stress around the bore aperture, the metal gasket is required to have means for balancing minutely the sealing stress throughout around the bore aperture.
There is no prior metal gasket in which the auxiliary plate or shim varies in radial width circumferentially around the bore aperture to compensate for variations in sealing stress, with taking into consideration a correlation between the sealing stress and the radial width of the shim. In the metal gasket in which a shim is laid on a bead plate, moreover, the shim uniform in radial width throughout around the bore aperture is subjected to such sealing stress distribution that the stress, though becoming high at areas neighboring the tightening holes, lowers at other areas remote from the tightening holes. That is to say, the metal gasket squeezed between the cylinder head and the cylinder block experiences much tightening force and correspondingly much affection of explosion in the engine at the areas nearby the tightening holes in which the hold-down bolts fit to clamp the metal gasket. Thus, the sealing stress around the bore apertures also inevitably becomes much higher at the areas neighboring the tightening holes than at the residual areas. After all, there arises a major problem in which the sealing stress occurring in the metal gasket can not kept uniform or constant throughout around the bore apertures.
Latest engines require low rigidity or/and high combustion pressure, but they frequently involve a major problem to be solved, in which minute variations in the sealing stress occurring circumferentially around the bore apertures has an influence undesirable from the view point of sealing performance of the metal gasket. That problem is much liable to arise in, especially, the metal gasket that disposed between the confronting cylinder head and block of the closed-deck type engine, in which the tightening load is selected at somewhat high level.
The present invention has as its primary object to overcome the problem described just above and, especially, the provision of a metal gasket that may be preferably used in a closed-deck type engine, which is made compact or slim in construction, light in weight and also tough against high combustion pressure. More particularly, the metal gasket of the present invention is simple in construction and composed of a bead plate and a shim laid on the bead plate, the shim being changed in radial width circumferentially to partly compensate for a sealing stress distribution around a bore aperture formed in the bead plate. The radial width of the shim laid on the bead plate is made varied continuously around the bore aperture in the bead plate, thereby balancing minute variations in sealing stress to help ensure the sealing pressure distribution that is uniform circumferentially of the bore aperture.
The present invention is concerned with a metal gasket adapted to be used interposed between mating surfaces of a cylinder block and cylinder head held together with tightening bolts, which comprises a bead plate made with first bore apertures and beads extending around the first bore apertures, each to each aperture, and a shim made with second bore apertures in alignment with the first bore apertures and laid on the bead plate, the shim being composed of areas extending around the second bore apertures and bridges connecting the adjoining areas with each other; the improvement wherein the shim around the second bore apertures has a radial width at the bridges between any adjoining second bore apertures, which is substantially equal with a width across a web defined around any cylinder bore formed in mating surfaces of the cylinder block and cylinder head, and the shim at areas other than the bridges is varied continuously in its radial width in a range of from a flat area around the first bore apertures in the bead plate to an area corresponding the bead on the bead plate, whereby variations in radial width of the shim around the second bore apertures result in compensating partly for a sealing stress distribution around the first bore apertures.
In accordance with an aspect of the present invention, an improved metal gasket is disclosed, wherein the shim is defined such that its area around the second bore apertures, when relieved from tightening load, extends in radial width from peripheral edges of the second bore apertures to a radial area covering at least an range corresponding any bead nearest the first bore apertures, while under tightening load extends over inside roots of the beads even at its minimum radial width.
In accordance with another aspect of the present invention, an improved metal gasket is disclosed, wherein the shim around the second bore apertures has a radial width that is made minimum at areas neighboring tightening holes formed in the bead plate for fitting the bolts.
In the metal gasket of the present invention constructed as described just above, the sealing stress occurring at the beads extending around the bore apertures may be properly, easily compensated or regulated with only changing the radial width of the shim laid on the bead plate at the areas around the bore apertures, without requiring altering partly the thickness of the shim and/or the height of the bead raised on the bead plate. The shim provided in the present invention is successful in making the sealing stress distribution around the bore aperture regulate minutely along the periphery of the bore aperture, thereby compensating easily for the unbalanced distribution of the sealing stress about the bore aperture to keep the desired sealing stress uniform circumferentially of the bore aperture. That is to say, according to the metal gasket of the present invention, the proper regulation of the sealing stress happening in the metal gasket may be easily realized by merely varying partly the radial width of the shim along the periphery of the bore aperture.
Generally speaking about most metal gaskets, the tightening force of bolts to clamp together the cylinder block and cylinder head makes much exertion on the bead plate at the areas neighboring the tightening holes for the bolts. Accordingly, the reduction of the shim in radial width at the areas neighboring the tightening holes results in decreasing the sealing stress happening owing to the shim, thus allowing the sealing stress to render uniform throughout the metal gasket. This construction of the shim of the present invention is preferably applied to, especially, the closed-deck type engine to regulate the sealing stress in the metal gasket.
In another metal gasket in which the shim is disposed between the confronting bead plates, the shim has no irregularity on its own opposite surfaces and, therefore, makes it possible to reduce overall height or thickness of the gasket and also to keep small the variations in the clearance between the cylinder block and cylinder head, which might happen owing to the pressure inside the cylinder bore. In addition, the metal gasket of the present invention, as having the shim in place of folded portions, is simple in construction.